Composite for providing a rigid-flexible circuit board construction and method for fabrication thereof

ABSTRACT

A composite for providing a rigid-flexible circuit board includes at least one core that contains a dielectric substrate and at least one conductive layer thereon; at least one sub-composite that includes a polyimide and a rigid dielectric substrate; a release layer; at least one rigid bonding layer and a second conductive layer. The composite is used in the fabrication of a rigid-flexible circuit board.

TECHNICAL FIELD

The present invention is concerned with a composite for providing arigid-flexible circuit board construction. In addition, the presentinvention is concerned with a method for the fabrication of arigid-flexible printed circuit board. The present inventionsignificantly reduces problems associated with excessive thermalexpansion of certain constituents of the circuit board as experienced inprior art configurations. The present invention provides for compactflexible multi-layer circuits.

BACKGROUND ART

Printed circuit boards find a wide variety of uses in the electronicsindustry with the demand for high performance, printed wiring, orcircuit boards for various applications steadily increasing. Forinstance, the complexity, compactness and electrical performancerequirements of printed circuit boards have significantly increased overthe last few years. To meet the demands of high performance along withminimum space and weight requirements, both multi-layer and rigid-flexcircuits have been employed. The rigid-flex printed circuit boardstypically are made by individual flexible layers and rigid layerslayed-up together to form a multi-layer construction.

The flexible layers are typically an integral part of both the rigidportions and flexible portions of the printed circuit board. The rigidlayers are part of only the rigid segments of the printed circuitboards. A typical material used for the flexible layers is the polyimideavailable under the trade designation Kapton from DuPont as thedielectric polymer film. In addition, the flexible layer typicallyincludes an acrylic adhesive covering the Kapton in order to secureadequate bonding to the core or base dielectric and conductive layersuch as copper.

The rigid segments of the rigid-flex printed circuit boards aretypically formed of reinforced epoxy or reinforced polyimide laminate.In addition, acrylic adhesive is also typically used to bond the rigidand flexible layers together. Although, the acrylic adhesive exhibitsexcellent flexibility and heat resistance, problems have beenencountered due to its high coefficient of thermal expansion as comparedto copper, the dielectric core, and solder such as tin/lead solder thatmight be present in attaching integrated circuit chips to the board.These problems occur when the constructions are subjected to acceleratedthermal cycling where the acrylic adhesive expands and contracts at avery high rate as compared to the other materials employed infabricating the printed circuit board. The high expansion creates agreat deal of stress in plated through holes and results in cracking inthe walls of the through holes.

One attempt to overcome this problem is described in U.S. Pat. No.4,800,461 to Dixon et al. This patent refers to a multi-layer rigid flexprinted circuit board obtained by providing structures having rigidsections that incorporate materials which when subjected to elevatedtemperatures do not expand sufficiently in the Z direction to causeproblems such as de-lamination and cracking of plated through holes. Theflex section of these boards include flexible material that extend lessthan 35 mils in the rigid section and does not penetrate throughout therigid section of the board. The insulator material of the flex segmentis typically constructed of Kapton having an acrylic adhesive thereonfor adhesion to the dielectric core and the small segment of overlapwith the rigid segments of the board. However, such construction is notentirely satisfactory since the presence of the acrylic adhesive stillremains in the flexible segments of the board and to some extent,although significantly reduced, in the rigid segments of the boards.Accordingly, the problems associated with high thermal expansion arestill not entirely eliminated in such configuration.

SUMMARY OF INVENTION

The present invention provides composites that are suitable forfabricating rigid-flexible multi-layer circuit boards which overcomeproblems associated with the high thermal expansion due to the acrylicor similar adhesives. In fact, the composite of the present inventiondoes not require the use of an acrylic or similar type adhesive in anyportion of the printed circuit board including the rigid and flexiblesegments thereof. The characteristics achieved from circuit boardspursuant to the present invention with respect to performance, space andweight requirements, render the boards especially suitable for miniatureelectronic packaging.

More particularly, the present invention is concerned with a compositefor providing a rigid-flexible circuit board construction that includesat least one core; at least one sub-composite; at least one releaselayer; at least one rigid bonding layer; and at least one conductivelayer.

The core comprises a dielectric substrate and at least one conductivelayer located on the dielectric substrate. The sub-composite comprises apolyimide and a rigid dielectric substrate. The sub-composite is locatedadjacent the at least one conductive layer of the core. The at least onerelease layer is located adjacent the at least one sub-composite and islocated in a predetermined location to generally correspond to thedesired flexible segment of the circuit board. Located adjacent to bothsides of the at least one release layer is at least one rigid bondinglayer that is to correspond to the desired rigid portion of the circuitboard. The at least one second conductive layer is located adjacent theat least one bonding layer and the at least one release layer.

In addition, the present invention is concerned with a rigid-flexiblecircuit board construction that comprises at least one core; at leastone sub-composite; at least one rigid bonding layer; and at least onesecond conductive layer.

The at least one core includes a dielectric substrate and at least oneconductive layer located on the dielectric substrate. The sub-compositeis located adjacent the at least one conductive layer of the core and islocated in both the rigid and flexible segments of the circuit board.The sub-composite includes a polyimide and a rigid dielectric substrate.The at least one bonding layer is located adjacent the at least onesub-composite and at the rigid segments of the printed circuit board butnot at the flexible segments thereof. The at least one second conductivelayer is located adjacent the at least one rigid bonding layer.

The present invention is also concerned with a process for fabricating arigid-flexible circuit board. The process includes providing at leastone core that includes a dielectric substrate and at least oneconductive layer located on the dielectric substrate. At least onesub-composite comprising a polyimide and a rigid dielectric substrate isprovided and is placed adjacent the at least one conductive layer inthose segments of the circuit board that will subsequently form therigid and flexible segments of the circuit board. A release layer isprovided adjacent the at least one sub-composite in a location tocorrespond generally to that segment of the circuit board which is to bethe flexible segment of the circuit board. Adjacent the ends of therelease layer is provided at least one rigid bonding layer located inthose segments of the circuit board that are to provide the rigidsegments of the circuit board but not at those segments which are toprovide the flexible segments of the circuit board. A second conductivelayer is provided adjacent the at least one rigid bonding layer tothereby form a composite.

The composite is then laminated. Portions of the second conductive layerthat correspond to the subsequently to be formed flexible segment of thecircuit board are removed and then the release layer is removed tothereby provide the desired rigid-flexible circuit board.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1-3 are schematic representations of the composite of the presentinvention during various stages of fabrication.

FIG. 4 illustrates a composite employing a plurality of cores andrelease layers pursuant to the present invention.

BEST AND VARIOUS MODES FOR CARRYING-OUT INVENTION

In order to facilitate an understanding of the present invention,reference is made to the figures wherein the same numerals in differentfigures refer to the same or corresponding structures. Reference to FIG.1 illustrate a core member 1 that comprises a dielectric substrate 2having at least one and preferably two opposed conductive layers thereonsuch as copper. The dielectric substrate 2 of the core is typically acured fiberglass reinforced epoxy resin composition, more typically anFR-4 composition including those disclosed in U.S. Pat. Nos. 3,523,037,4,597,996, disclosures of which are incorporated herein by reference.The thickness of the dielectric portion is typically about 3 mils toabout 6 mils and preferably about 3.6 mils to about 5.6 mils. In aspecific embodiment the dielectric substrate of the core is composed oftwo sheets, each being 1.8 mils, laminated together. In place of anepoxy, the dielectric of the core can be a polyimide film (e.g.--UpilexSGA available from UBE Ltd.) Such is a polyimide obtained frombiphenyltetracarboxylic dianhydride and a diamine. Such has a typicalrepeating unit as follows: ##STR1##

Moreover, the dielectric substrate of the core can be a flexiblenon-reinforced substrate such as a composite of Upilex SGA (e.g.--about1-2 mils) coated on the surface or surfaces that are to contact theconductive layer with an epoxy (e.g.--about 0.5 to about 0.75 mils) suchas those disclosed above. The epoxy provides for a tenacious bond withthe conductive layer such as the copper.

A typical FR-4 epoxy composition contains 70-90 parts of brominatedpolyglycidyl ether of bisphenol-A and 10-30 parts oftetrakis(hydroxyphenyl) ethane tetraglycidyl ether cured with 3 to 4parts of dicyandiamide, and 0.2 to 0.4 part of a tertiary amine, allparts being parts by weight per 100 parts of resin solids.

Another typical FR-4 epoxy composition contains:

(a) about 25 to about 30 parts by weight of a tetrabrominated diglycidylether of bisphenol-A having an epoxy equivalent weight of about 350 toabout 450;

(b) about 10 to about 15 parts of weight of a tetrabrominated diglycidylether of bisphenol-A having an epoxy equivalent weight of about 600 toabout 750; and

(c) about 55 to about 65 parts by weight of at least one epoxidizednonlinear novalak having at least six terminal epoxy groups, along withsuitable curing and/or hardening agents.

Another typical FR-4 epoxy composition contains 70-90 parts ofbrominated polyglycidyl ether of bisphenol-A and 10-30 parts of tetrakis(hydroxyphenyl) ethane tetraglycidyl ether cured with 0.8-1 phr of2-methylimidozale. Still other FR-4 epoxy compositions employtetrabromobisphenol A as the curing agent along with 2-methylimidazol asthe catalyst.

The conductive layer 3 is preferably copper and typically has athickness of about 0.5 to about 2.8 mils and more typically about 1.0 toabout 1.4 mils. In addition according to preferred aspects of thepresent invention, the conductive layer comprises a layer of chromiumcontacting the dielectric followed by a layer of copper. The chromium istypically about 200 angstroms to about 700 angstroms thick and moretypically about 300 angstroms to about 500 angstroms thick with thecopper being about 0.5 mil to about 2.8 mil thick and more typicallyabout 1.0 mil to about 1.4 mil thick. The conductive layer 3 ispatterned by well known lithographic techniques to form desiredcircuitry thereon.

Adjacent the conductive layer 3 of core 1 is provided at least onesub-composite. The sub-composite comprises a polyimide layer 4 and arigid dielectric substrate layer 5. The preferred polyimide layer 4 isUpilex SGA and the rigid dielectric substrate layer 5 is preferably anepoxy composition such as an epoxy composition described in U.S. Pat.Nos. 3,523,037 and 4,597,996. The epoxy composition at this stage is inthe B-stage which is about 30% to about 50% cross-linked.

The polyimide is typically about 0.5 mil to about 5 mils and moretypically about 0.8 mil to about 2 mils. The rigid dielectric substrate5 is about 0.5 mil to about 1 mil and more typically about 0.5 mil toabout 0.75 mils. The sub-composite is provided adjacent the conductivelayer 3 on the dielectric substrate and is provided in those regionswhich are to be subsequently used as both the rigid and flexible regionsof the printed circuit board. In addition, the sub-composite can beprovided as a previously combined unit of the polyimide and epoxypolymer or can be applied as individual layers at this stage of theprocess if desired.

The epoxy polymer composition is adjacent the conductive layer 3 inorder to ensure a tenacious bond between the sub-composite andconductive layer.

Next, a release layer 6 is provided adjacent the sub-composite. Therelease layer 6 is typically a material that is relatively non-adherentto the polyimide and to the subsequently to be applied layers or isreadily removable therefrom. Typical materials include fluorinatedpolymeric materials such as fluorinated ethylene/propylene copolymers,polymers of chlorotrifluoroethylene, hexafluoropropylene polymers,polyvinylidene fluoride and polytetrafluoroethylene andpolyfluoroalkoxyethylene.

A preferred release layer comprises a metal such as copper or brass toprovide sufficient rigidity or stability coated with a relativelynon-adhering material such as polytetrafluoroethylene or preferablypolyfluoralkoxyethylene. The release layer 6 is located only in thatarea which corresponds generally to the segment which is to provide theflexible segment of the printed circuit board.

The release layer is typically about 5 to about 15 mils and moretypically about 9.6 mils. In a preferred embodiment, the release layercomprises 1 sheet of copper about 5.6 mils thick and clad on each sidewith 2 mils of a polyfluoroalkoxyethylene. The release layer is criticalto the success of the present invention to provide for instance adequatepressure at the flexible locations of the circuit board.

Adjacent to each end of release layer 6 is at least one rigid bondinglayer 7. Typical bonding layers 7 are glass fiber reinforced epoxypolymer compositions such as those described in U.S. Pat. Nos. 3,523,037and 4,597,996. The rigid bonding layers at this stage in the process arein the B-stage. Alternatively, the bonding layer can be a combination ofa polyimide such as Upilex SGA coated both sides with the B-stagedepoxy. Preferably the thickness of the rigid bonding layer issubstantially equal to the thickness of the release layer and preferablyis comprised of two separate layers 7' and 7" as illustrated in FIG. 1.

The rigid bonding layer is to be present in those portions of thecomposite which is to provide for the rigid segments of the printedcircuit boards but not in those portions which are to provide theflexible segments of the printed circuit board.

A second conductive layer 8 is provided adjacent the rigid bonding layer7 and the release layer 6. The second conductive layer is typicallyabout 0.7 to about 1.4 mils thick.

Next, the assembly is laminated under pressure and heat to provide alaminated composite. Typically, the lamination is carried out at atemperature of about 340° to about 390° F. and more typically at about350° to about 375° F. and preferably at about 350° F. and at pressuresof about 200 to about 700 psi and more typically at pressures of about300 to about 500 psi and preferably at about 350 psi. The lamination isgenerally continued for about 60 to about 120 minutes, typical of whichis about ninety (90) minutes.

After the lamination is completed, the external circuitry on conductivelayer 8 is provided as well as plated through holes 9. The structure canthen be cleaned and any components to be connected to the rigid segmentsof the printed circuit board can be provided.

Next, that portion of the conductive layer 8 as well as any other layerslocated above spacer 6 which correspond to that portion of the compositewhich is to provide the flexible segments of the printed circuit boardare removed. For instance, such materials can be removed by controlleddepth milling. A typical milling procedure can be carried out using aPosalux or Radol milling machine.

Next, the release layer is removed.

In the preferred aspects of the present invention, release layer 6 canbe removed by flexing the structure and permitting release layer 6 tomerely fall out. In less preferred embodiments, when the release layer 6adheres to an adjacent layer such as conductive layer 8, it can beremoved by physical means such as milling or possibly by etching.

The flexible-rigid printed circuit board obtained is schematicallyillustrated in FIG. 3.

FIG. 4 represents a composite prior to lamination employing a pluralityof cores (three (3) cores) with interim release layers 6 and bondinglayers 7. The composite is processed in the same manner as describedabove for that shown in FIGS. 1 and 2 except that in the removal stepprior to removal of the spacer, that region designated as R is removed.

As noted above, the present invention makes it possible to provide arigid-flexible printed circuit board that does not require an acrylic orother adhesive layer having relatively high thermal expansion of above300 ppm/°C. and especially in the rigid segments and thereby avoidsproblems associated in the prior art due to the high thermal expansionof such. However, although not required or even desired, acrylic orsimilar adhesives can be tolerated in the flexible segments withoutadversely affecting the composite to an undesired extent. Moreover, thisis achieved pursuant to the present invention in a manner that employsrelatively little real estate thereby providing a very compact printedcircuit board that is especially suitable in miniature electronicpackaging.

What is claimed is:
 1. A composite for providing a rigid-flexiblecircuit board construction comprising at least one core wherein said atleast one core comprises both a dielectric substrate and at least oneconductive layer located on said dielectric substrate; at least onesub-composite located on top of said at least one conductive layer andwhich comprises a polyimide layer and a rigid dielectric substrate, andwherein said rigid dielectric substrate contacts said at least oneconductive layer; a release layer located on top of said at least onesub-composite in a predetermined location to correspond to the desiredflexible segment of said circuit board; at least one rigid bonding layerlocated next to each of the two sides of said release layer tocorrespond to the desired rigid portion of said circuit board; and atleast one second conductive layer located on top of said at least onebonding layer and said at least one release layer.
 2. The composite ofclaim 1 wherein said dielectric substrate of the core is a reinforcedepoxy composition or polyimide film.
 3. The composite of claim 1 whereinsaid dielectric substrate of the core is a flexible substrate.
 4. Thecomposite of claim 3 wherein said flexible substrate comprises acombination of at least one layer of a polyimide film and at least onelayer of an epoxy composition and wherein said epoxy compositioncontacts said at least one conductive layer.
 5. The composite of claim 4wherein said polyimide is obtained from biphenyltetracarboxylicdianhydride and a diamine.
 6. The composite of claim 2 wherein saidpolyimide is obtained from biphenyltetracarboxylic dianhydride and adiamine.
 7. The composite of claim 1 wherein the polyimide of saidsub-composite is obtained from biphenyltetracarboxylic dianhydride and adiamine; the rigid dielectric substrate of said sub-composite is anepoxy composition wherein said epoxy composition contacts saidconductive layer of said core.
 8. The composite of claim 1 wherein saidrelease layer comprises a metal substrate coated with a fluorinatedpolymer.
 9. The composite of claim 8 wherein said metal substrate iscopper.
 10. The composite of claim 1 wherein said at least one rigidbonding layer is a reinforced epoxy composition.
 11. The composite ofclaim 10 wherein the thickness of said at least one rigid bonding layeris substantially equal to the thickness of said release layer.
 12. Thecomposite of claim 1 wherein said second conductive layer is copper. 13.The composite of claim 1 wherein all bonding material has a thermalexpansion below 300 ppm/°C.
 14. A rigid-flexible circuit boardconstruction comprising:(a) at least one core, wherein said at least onecore comprises both a dielectric substrate and at least one conductivelayer located on said dielectric substrate; (b) at least onesub-composite located on top of said at least one conductive layer andin both the rigid and flexible segments of said circuit board and whichcomprises a polyimide layer and a rigid dielectric substrate, andwherein said rigid dielectric substrate contacts said at least oneconductive layer; (c) at least one rigid bonding layer located on top ofsaid at least one sub-composite and at the rigid segments of said boardbut not at the flexible segments of said board; and (d) a secondconductive layer on top of said at least one rigid bonding layer. 15.The construction of claim 14 wherein said dielectric substrate of thecore is a reinforced epoxy composition or polyimide film.
 16. Theconstruction of claim 14 wherein said dielectric substrate of the coreis a flexible substrate.
 17. The construction of claim 16 wherein saidflexible substrate comprises a combination of at least one layer of apolyimide film and at least one layer of an epoxy composition andwherein said epoxy composition contacts said at least one conductivelayer.
 18. The construction of claim 17 wherein said polyimide isobtained from biphenyltetracarboxylic dianhydride and a diamine.
 19. Theconstruction of claim 14 wherein the polyimide of said sub-composite isobtained from biphenyltetracarboxylic dianhydride and a diamine; therigid dielectric substrate of said sub-composite is an epoxy compositionwherein said epoxy composition contacts said conductive layer of saidcore.
 20. The construction of claim 14 wherein said at least one rigidbonding layer is a reinforced epoxy composition.
 21. The construction ofclaim 14 wherein said second conductive layer is copper.
 22. Theconstruction of claim 14 wherein all bonding material has a thermalexpansion below 300 ppm/°C.
 23. The construction of claim 15 whereinsaid polyimide is obtained from biphenyltetracarboxylic dianhydride anda diamine.
 24. The composite of claim 1 comprising 3 cores whereinlocated both above and below the middle one of said 3 cores is asub-composite, and located above and below the two outer cores is atleast one rigid bonding layer.
 25. The construction of claim 14comprising 3 cores wherein located both above and below the middle oneof said 3 cores is a sub-composite, and located above and below the twoouter cores is at least one rigid bonding layer.